NORWICH, UK: It is well known that climate-induced sea level rise is a major threat. What is less known is the threat of sinking the land. And in many of the most densely populated coastal areas, land is sinking faster than sea levels are rising.
Parts of Tokyo for example sank 4m during the 20th century, with 2m or more sinking reported in Shanghai, Bangkok, and New Orleans. This process is known as subsidence.
Slow subsidence occurs naturally in river deltas, and can be accelerated by the withdrawal of groundwater, oil or gas causing the soil to consolidate and the surface to lose altitude.
Land subsidence causes relative sea level rise (sea level rise plus land sinking). It turns out that agricultural land is becoming salty, damaging buildings, causing widespread flooding that can even mean the loss of the entire coastal area.
Land subsidence can threaten flooding in low-lying coastal areas, much more than sea level rise, but scientists are only just becoming aware of the global implications of the threat with regard to coastal cities.
In fact, although the average coastal area is experiencing a relative sea level rise of less than 3mm per year, the average coastal population is experiencing an increase of about 8mm to 10mm per year. This is because a lot of people live in the delta and especially the cities in the delta that are receding.
That’s a key finding from our new study, in which we analyze how rapidly cities are sinking around the world and compare this to global land subsidence data including less densely populated coastlines.
Our findings reflect that people often choose to live in river deltas, floodplains and other areas that are already prone to sinking, and thus further increase land subsidence.
In particular, receding cities host more than 150 million people in the coastal zone – that’s roughly 20 percent of the world’s people who live by the sea. This means that relative sea level rise will have a more sudden and more severe impact than scientists previously thought.
Here are some of the cities most affected:
The Indonesian capital Jakarta is home to 10 million people, and is built on a lowland next to the sea. The withdrawal of groundwater caused the city to sink more than 3m from 1947 to 2010 and large parts of the city still sink 10cm or more each year.
Land subsidence does not occur evenly, creating uneven risks that make urban planning difficult. Buildings are now flooded, cracks appear in abandoned infrastructure.
Jakarta has built a taller sea wall to compensate for land subsidence. However, as groundwater pumping continues, this patching policy can only last long before the same problem recurs. And cities need to keep pumping because groundwater is used for drinking water. Fetching water, which is essential for human survival, ultimately puts people at risk of waterlogging.
The battle against land subsidence is slowly disappearing, with the government proposing in 2019 to move the capital to a purpose-built city on the island of Borneo more than 1,000 km away, with land subsidence being one of many reasons.
Thriving in the last few decades, and now with a population of 26 million, Shanghai is another shit. The city has a maximum subsidence rate of about 2.5 cm per year. Again this is largely due to lowering the groundwater table, in this case thanks to drainage for building skyscrapers, metro lines and highways (eg Metro Line 1, built in the 1990s, led to rapid subsidence).
If no additional cover is built, by 2100 this rate of sea level decline and rise means that storm surges could overwhelm about 15 percent of the city.
In New Orleans, embankments and ditches over the centuries have effectively drained the city and submerged it, leaving about half below sea level.
When Hurricane Katrina hit the embankment in 2005, the city didn’t stand a chance. The hurricane caused at least US $ 40 billion in damage and particularly affected the city’s African American community. More than 1,570 people died across the state of Louisiana.
If the city did not recede, the damage would be greatly reduced and lives would be saved.
Decisions made decades or more ago paved the way for the catastrophe seen today, and what we will see in the future.
THERE IS NO SIMPLE SOLUTION
So what can be done? Building a sea embankment or embankment is one of the immediate solutions. This of course stops water getting in, but remember that the sea wall is sinking too, so it has to be extra large to be effective in the long term.
In urban areas, engineers can’t lift the ground easily: It can take decades as buildings and infrastructure are updated. There is no simple solution, and large-scale urban land subsidence is largely irreversible.
Several cities have found “solutions”. Tokyo, for example, succeeded in stopping land subsidence from around 1960 onwards thanks to stronger regulations on water pumping, but it could not eliminate the overall risk as parts of the city are below sea level and depend on embankments and pumps to be habitable.
Indonesia’s bold proposal to move its capital city may be the ultimate solution.
Increased urbanization especially in delta areas and freshwater demand means land subsidence will remain a pressing issue in the coming decades.
Addressing land subsidence is complementary to addressing climate-induced sea level rise and both need to be addressed. The combination of rising sea levels and sinking land will further endanger coastal cities.
Hear how changes in the oceans are causing sea levels to rise in this episode of The Climate Conversations:
Sally Brown is a scientist at Bournemouth University and Robert James Nicholls is professor of Climate Adaptation, University of East Anglia. This comment first appearance on The Conversation.